Mark R. Stoudt

SUPPRESSION OF FATIGUE CRACKING WITH NANOMETER-SCALE MULTILAYERED COATINGS

Department of Materials Scienceand Engineering Department, Johns Hopkins University, Baltimore, MD 21218(Received March 1, 2000)(Accepted in revised form May 4, 2000)Keywords: Fatigue; Multilayers; Plating; Surfaces; Crack initiationIntroductionFatigue crack initiation in initially smooth samples of homogeneous ductile metals almost alwaysoccurs at the free surface as a result of surface roughening and the development of a critical surfacemorphology (1–3). For face centered cubic (FCC) metals with high stacking fault energies such as Cu,this critical morphology is the result of irreversible dislocation processes occurring during cyclic slipand consists of notches and peaks (intrusions and extrusions) formed at persistent slip bands (PSBs)(2,4). Recognizing that it should be possible to influence fatigue crack initiation by modifying theproperties or microstructure of the surface, investigators have examined the influence of variouscoatings and surface treatments such as shot peening, anodizing, oxidizing, nitriding, boriding, plasmaspraying, ion beam mixing, and ion implanting (5–10). While hard coatings prevent surface rougheningand crack initiation by the normal roughening mechanism, they usually result in modest improvementsbecause cyclic loading manages to initiate fatigue cracks by another mechanism. For example,Hornbogen and Verpoort (9) examined the influence of boriding, nitriding, and plasma spraying onfatigue crack initiation in austenitic stainless steels and found that while these coatings suppressedsurface roughening, fracture of the films resulted in slip localization and rapid initiation of fatiguecracks which actually degraded performance in some cases (9). Alden and Backofen (5) examined thickanodic films on Al and found that they could extend the fatigue life by periodically treating the surfaceto remove cracks in the films. This review of the literature (1–13) indicates that a surface film shouldhave five properties to maximize fatigue crack initiation resistance: (i) hardness - to prevent surfaceroughening, (ii) ductility (toughness) - to prevent cracking where PSBs intersect the film, (iii) cyclicwork hardenability - to prevent slip localization, (iv) residual compressive stresses - to reduce themagnitude of tensile stresses in the film, and (v) adherence - to stay on the substrate.The mechanical properties of nanometer-scale multilayered thin film coatings can be modified andadjusted by changing the composition and thickness of the layers (14–16). As a result, multilayeredcoatings can be developed that are optimized with respect to the properties identified above as beingdesirable in a coating for fatigue crack initiation resistance. In particular, a multilayer composed of thinlayers of two normally ductile FCC metals may have sufficient hardness to block surface slip yet retainsufficient ductility, toughness, and cyclic work hardening capacity, that cracking of the film or other sliplocalization mechanisms will not prevent this film from suppressing fatigue crack initiation. The

Homogenization kinetics of a nickel-based superalloy produced by powder bed fusion laser sintering

Additively manufactured (AM) metal components often exhibit fine dendritic microstructures and elemental segregation due to the initial rapid solidification and subsequent melting and cooling during the build process, which without homogenization would adversely affect materials performance. In this letter, we report in situ observation of the homogenization kinetics of an AM nickel-based superalloy using synchrotron small angle X-ray scattering. The identified kinetic time scale is in good agreement with thermodynamic diffusion simulation predictions using microstructural dimensions acquired by ex situ scanning electron microscopy. These findings could serve as a recipe for predicting, observing, and validating homogenization treatments in AM materials.

A combined study of surface roughness in polycrystalline aluminium during uniaxial deformation using laser-induced photoemission and confocal microscopy

A new technique integrating measurements obtained by photo-stimulated electron (PSE) emission and scanning laser confocal microscopy (SLCM) has been developed to characterize the deformation of commercially pure aluminium in uniaxial plastic strain. Real time, in situ PSE signals provide details about the evolution and propagation rates of surface events during the deformation process. High-resolution SLCM measurements yield details describing the relative magnitude and spatial distribution of the surface features. During homogeneous deformation, uniform generation of surface area produces a monotonic increase in PSE intensity whereas necking induces a saturation condition. Analysis of the surface area created by the deformation revealed that the rate of generation correlates well with the PSE intensity. The consistencies in the data acquired with these two techniques signify that they yield complementary information and that the combination provides essential details about the deformation process in a material with low hardness such as an aluminium alloy.

Influence of Postbuild Microstructure on the Electrochemical Behavior of Additively Manufactured 17-4 PH Stainless Steel

The additive manufacturing build process produces a segregated microstructure with significant variations in composition and phases that are uncommon in traditional wrought materials. As such, the relationship between the postbuild microstructure and the corrosion resistance is not well understood. Stainless steel alloy 17-4 precipitation hardened (SS17-4PH) is an industrially relevant alloy for applications requiring high strength and good corrosion resistance. A series of potentiodynamic scans conducted in a deaerated 0.5-mol/L NaCl solution evaluated the influence of these microstructural differences on the pitting behavior of SS17-4. The pitting potentials were found to be higher in the samples of additively processed material than in the samples of the alloy in wrought form. This indicates that the additively processed material is more resistant to localized corrosion and pitting in this environment than is the wrought alloy. The results also suggest that after homogenization, the additively produced SS17-4 could be more resistant to pitting than the wrought SS17-4 is in an actual service environment.

Formation of Nb-rich droplets in laser deposited Ni-matrix microstructures

Abstract Ni-rich γ cells and Nb-rich eutectic droplets that form during laser power bed fusion solidification of Ni-Nb alloys are studied using experiments and simulations. Finite element simulations estimate the cooling rates in the melt pool and phase-field simulations predict the resulting cellular microstructures. The cell and droplet spacings are determined as a function of cooling rate and fit to a power law. The formation of Laves phase is predicted for a critical composition of Nb in the liquid droplets. Finally, our simulations demonstrate that anisotropy in the γ orientation influences the Laves fraction significantly.

Fundamental relationships between deformation-induced surface roughness, critical strain localisation and failure in AA5754-O

This research employs two approaches to characterise the apparent structure observed in localised strain maps constructed from surface topography data acquired from AA5754-O sheet stock that was deformed in three in-plane stretching modes. The first uses a conventional two-point autocorrelation function (ACF), while the second uses the combination of the eigenvalue spectrum associated with each map and information theory. The results from the ACF analysis are inconclusive, implying that this technique lacks the sensitivity necessary to quantify the relationships between multi-point clustering and strain localisation. The information theory-based approach reveals that the relative spectral entropy increases monotonically, attains a maximum and then decreases sharply to the failure strain. This behaviour occurs in all three strain modes and results from two competing processes: one where the formation of structure is favourable and one where it is not. The crossover point is a clear indicator of the onset of critical strain localisation and, therefore, can be regarded as a precursor to failure because once the dominant process shifts, additional strain results in the precipitate formation of a critical strain localisation event.

Evaluating the Relationships Between Surface Roughness and Friction Behavior During Metal Forming

The inhomogeneous distribution of surface asperities generated by deformation induces variability in the friction and initiates strain localizations during metal forming. The friction literature generally does not account for the strong influence localized variations in material properties have on the friction behavior. A prototype apparatus was developed that measures the friction behavior under simulated forming conditions and enables detailed characterization of the influences of the microstructure and the topographical conditions that occur under those conditions. The results demonstrate that the measurement system can resolve subtle real-time changes in the dynamic friction coefficient, and that a correlation could exist between the largest surface asperities and the largest variations in the measured friction coefficient.

Interlaboratory Study for Nickel Alloy 625 Made by Laser Powder Bed Fusion to Quantify Mechanical Property Variability

Six different organizations participated in this interlaboratory study to quantify the variability in the tensile properties of Inconel 625 specimens manufactured using laser powder bed fusion-additive manufacturing machines. The tensile specimens were heat treated and tensile tests were conducted until failure. The properties measured were yield strength, ultimate tensile strength, elastic modulus, and elongation. Statistical analysis revealed that between-participant variability for yield strength, ultimate tensile strength, and elastic modulus values were significantly higher (up to four times) than typical within-participant variations. Only between-participant and within-participant variability were both similar for elongation. A scanning electron microscope was used to examine one tensile specimen for fractography. The fracture surface does not have many secondary cracks or other features that would reduce the mechanical properties. In fact, the features largely consist of microvoid coalescence and are entirely consistent with ductile failure.

Topographic analysis and Weibull regression for prediction of strain localisation in an automotive aluminium alloy

Abstract We fit a two-parameter Weibull regression model by maximum likelihood estimation (MLE) to filtered surface roughness data. These data were acquired with scanning confocal laser microscopy performed on aluminium alloy AA 5754-O surfaces that were subjected to a range of plastic strain intensities in three different in-plane strain modes. Noting that one of the two Weibull regression parameters is, to a good approximation, invariant with strain intensity for a given strain mode, the authors find that the variation of the second parameter with strain intensity conforms to a simple quadratic function. These functions may then be used to generate accurate, statistically significant, single parameter predictors of both strain intensity and strain mode up to and including the onset of critical strain localisation and/or failure.

Topography of metallic surfaces subjected to plastic strain: Roughness, spatial correlations, and eigenvalue spectral entropy

This paper describes an analysis technique that integrates high-resolution topographical imaging and rigorous matrix-based three-dimensional characterization methods. We employ scanning confocal laser microscopy to obtain topographic images of the surface of an aluminum alloy subjected to various levels of uniaxial plastic strain. These images are discretized into a square array of pixels, each of which is then assigned a numerical value corresponding to the deviation of surface height relative to an averaged background. The result is a set of real, non-Hermitian n×n matrices, which are diagonalized to produce a collection of spectra, each of which consists of n complex eigenvalues. These eigenvalue spectra are observed to change systematically as the degree of plastic strain is varied. Because this approach is based solely on the behavior of the eigenvalue spectra, it eliminates the need for the a priori assumptions about surface character used in conventional topographic analyses. The information contai...